Why might I not be able to activate my drawing canvas grid guide?
It looks like Grid is the grid that is displayed when you are both in orthographic AND perpendicular view, it allows you to view the units when in ortho front view, ortho right view, etc... it always displays a grid perpendicular to your view, whereas Floor is the floor, it's only an horizontal plane, and it is seen whatever is your view, orthographic or not, but it's still an horizontal plane.

In your case the option is grayed because you are not in orthographic AND perpendicular view. It will appear when you'll be in both.